One approach to off-premise control of cable channels is to jam unauthorized channels with an inband carrier. This carrier can be moved rapidly from channel to channel, allowing one oscillator to jam several channels (agile jammer). The problem with this approach is that, as more channels are jammed, the time spent on each channel decreases, which decreases the masking ("scrambling") of the signals.
An alternative approach uses an oscillator for each of a number, N, of jammed channels. This oscillator is voltage tunable, and thus may be moved to any channel in a reasonable range. Once it is on its assigned channel, it must be kept there using frequency-lock or phase-lock techniques.
If a large number of oscillators (for example, eight to twelve) is required, individual frequency-lock control of each would be very costly.
U.S. Pat. No. 4,135,157, entitled "Pole Mounted Converter", discloses cable television converter means including a plurality of individual converter RF sections, each including an oscillator and a mixer. A plurality of subscriber channel selection means are connected, along with each of the oscillators to a digital tuning system. The digital tuning system sequentially and repetitively is connected to each subscriber selection means and its associated oscillator to provide tuning control during the period of such connection. Tuning control is provided by comparing the frequency of the oscillator with the selected channel oscillator frequency as provided by the subscriber selection means.
The following U.S. patents are noted of interest: U.S. Pat. No. 4,450,481, entitled "Tamper-Resistant Expandable Communications Systems", and U.S. Pat. No. 4,367,557, entitled "Wired Broadcasting Systems".